1. Technical Field
This invention relates to the field of autostereoscope hard copy imaging, and particularly to a method for generating such images using conventional photographic or electronic cameras. More particularly, the invention pertains to recording such images on film in such a manner that a three-dimensional depiction of the recorded scene is visible through an image projecting faceplate without the need for auxiliary optical viewing devices.
2. Background Art
Three-dimensional photography is comprehensively described in Three-Dimensional Imaging Techniques by Takanori Okoshi (New York: Academic Press, 1976, translated from the Japanese edition published in 1972) which provides a basis for describing the attributes and advantages of this invention. Okoshi initially distinguishes between truly three dimensional imaging and stereoscopic imaging on the basis of the amount of information involved. The quantity of information for a stereoscopic (or binocular) image is only twice that of a planar (one-dimensional) image, while much greater information is present for a truly three-dimensional image (which is often called an autostereoscopic image). Images of the latter type are truly spatial images that gradually show more of the right side of the object when the observer moves rightward, and more of the left side of the object when the observer moves leftward (which is often referred to as a "look around" capability). Integral photography is a method of recording a complete spatial image, that is, one viewable from a multiplicity of directions, upon a single flat photographic plate. The principles of integral photography were described by G. Lippman in 1908 in a paper read to the French Academy of Science. Integral photography thus has a long history of theoretical consideration and demonstration, but has enjoyed only limited commercial success.
Integral photography refers to the composition of the overall image as an integration of a large number of small photographic image components. Each photographic image component is viewed through a separate small lens usually formed as part of a mosiac of identical spherically-curved surfaces embossed or otherwise formed onto the front surface of a plastic sheet of appropriate thickness. The plastic sheet is subsequently bonded to or held in close contact with the emulsion layer containing the photographic image components. Lenticular photography is a special case of integral photography wherein the small lenses are formed as sections of cylinders running the full extent of the print area in the vertical direction. Recent commercial attempts at lenticular photography have included a multi-lensed 35 mm three-dimensional camera sold by Nimslo Corp., Atlanta, Ga., and a similar camera manufactured by Nishika Optical Systems, a division of Nishika Corp., Henderson, Nev. Though a sense of depth is clearly visible in prints made from these cameras, the resulting images have limited depth realism and appear to the viewer to "jump" as the print is rocked or the viewer's vantage relative to the print is changed.
The product of integral photography, that is, an integral photograph, can be further thought of as an X-Y array of microscopic slide projectors cascaded over the area of the print material. Each tiny lens, or lenslet, projects a microscopic view of the scene from a slightly different perspective than the one next to it. If the viewer's eye was concentrated on a singular lenslet surface, it would see only that portion of the view behind that lenslet which is angularly aligned with the line of sight to that lenslet. If the eye is moved laterally and continues to look at the same lenslet, it will see progressively different laterally angular portions of the view behind that lenslet. However, because the lenslets are made very small relative to the normal viewing distance, their apparent angular diameters may approach or subserve the angular resolution of the eye, with the result that features of the lenslets themselves are not apparent to the viewer, while the light emanating from them is.
The viewer then is able to mentally construct the entire array of optical beams from all lenslets into a recognizable scene without distraction from lenslet features. Since the right eye sees the array from a different vantage than the left eye, autostereoscopic depth perception is also present. By shifting the head laterally relative to the print surface, a changing autostereoscopic view is seen resulting in a "look around" capability which adds to the realism of the display. Integral photography also allows a "look around" capability in the vertical direction as well as the horizontal direction and an autostereoscopic view would also result if the print were rotated ninety degrees such that horizontal lines recorded from the original scene are now extending from bottom of the print to the top.
Since it is likely that most viewers prefer to view their photographs as models or reminders of the real world, it is not likely that they will choose to rotate the print for viewing. It was recognized as early as Lippman that instead of spherical lenslets, long cylindrical lenses extending from the top of the print to the bottom could be used to provide autostereoscopic views (and resultant "look around") in the horizontal direction only. This is sufficient to give a realistic three-dimensional model of the real world. Moreover, since vertical film space is not used to record alternative vertical views, the vertical detail recorded improves and approaches the film resolution limit, giving an improved impression of print quality. The long cylindrical lenses are called lenticules, and the principles of integral photography apply equally well to lenticular photography as long as one views the layouts or optical schematics in planes perpendicular to the cylindrical axis of the lenticules.
An optical method of making lenticular photographs is described by Okoshi in Chapter 4 of the aforementioned book. A photographic camera is affixed to a carriage on a slide rail which allows it to be translated in a horizontal direction normal to the direction of the desired scene. A series of pictures is taken wherein the camera is translated between subsequent exposures in equal increments from a central vantage point to lateral vantage points on either side of the central vantage point. The distance that the lateral vantage points are displaced from the central vantage point is dependent upon the maximum angle through which the lenticular material can project photographic image components contained behind any given lenticule before it begins to project photographic image components contained behind an adjacent lenticule. (It is not necessary to include a picture from the central vantage point, in which case the number of images will be even. If a picture from the central vantage point is included, the number of images will be odd.) The sum of the total number of views contained between and including the lateral vantage points will determine the minimum number of photographic components which eventually will be contained behind each lenticule.
In accordance with the Okoshi book, the negatives resulting from each of these views are then placed in an enlarger equipped with a lens of the same focal length as the camera lens. Since the camera had been moved laterally between successive exposures as previously described, the positions of the images in the original scene will be seen to translate laterally across the film format. Consequently, the position of the enlarged images from the negatives will also appear to move laterally with respect to the center of the enlarger's easel as successive negatives are placed in the film gate.
In making the print, an assemblage is made of a sheet of photographic material oriented with its emulsion side in intimate contact with the flat back side of a clear plastic sheet of appropriate thickness having lenticules embossed or otherwise formed into its front side. The assemblage is placed on the enlarger easel with the lenticular side facing the enlarger lens. The position of this assemblage on the easel is adjusted until the field of the central image is centered on the center of this assemblage, and an exposure of the information being projected out of the enlarger lens is made through the lenticules onto the photographic emulsion.
Subsequently, negatives from the successive exposures are placed in the film gate and the position of this assemblage is readjusted on the easel to reposition each respective view to the center of the assemblage, and additional exposures of the information being projected from the enlarger lens are made. When all the views contained between the lateral vantages have been exposed on the emulsion through the lenticular plastic sheet, the film sheet can be separated from the lenticular plastic sheet and developed. If the aperture of the enlarger lens is set to equal the amount of lateral shift between alternate views, the space behind each lenticule will be found to be exactly filled with photographic image components.
The final step in this process is to again reassemble the photographic film and the plastic sheet with intimate contact between the emulsion layer and the flat side of the lenticular plastic sheet, with the lenticular sheet so positioned laterally that the long strips of adjacent images resulting from exposures through the cylindrical lenticules are again positioned in a similar manner under the lenticules for viewing. This method of image recording is called an "indirect" technique because the final print recording is indirectly derived from a series of two-dimensional image recordings.
Ideally, an integral or lenticular photograph would display an infinite number of different angular views from each lenslet or lenticule. This would be impossible since each angular view must have a corresponding small finite area of exposed emulsion or other hard copy media whence is its source of display. Consequently, as an upper limit, the number of views must not exceed the resolution limit of the hard copy media, and, perhaps practically more significant, must not exceed the resolving power of the lenticules. In the aforementioned camera manufactured by Nimslo, the number of views behind each lenslet or lenticule was limited to four views, two of which were considered left perspective views and the remaining two were right perspective views. This was well below the resolution limit of the photographic emulsion and allowed for only two options for stereoscopic viewing perspectives as the viewer's head was moved laterally. Consequently, an unrealistic image jump results when the viewer's vantage moves relative to the separate views on the print and the viewing conditions for "inverse stereo" are increased wherein the right eye sees the image intended for the left eye and vice versa. (This methodology, however, allows for many more stereoscopic views. For example, the 1969 Annual Report to Stockholders of Eastman Kodak Company displays a lenticular photo comprising a large number of alternate views of the scene. The resulting print is much more effective than with fewer views.)
The concept of integral photography by an indirect technique is also described in U.S. Pat. Nos. 4,724,449 and 4,956,705, naming Douglas Wright as inventor, and assigned to Dimensional Visions Group of Philadelphia, Pa. U.S. Pat. No. 4,724,449 describes a photographic camera with a laterally shifting film holder to capture a number of perspectives of a scene and to record image information onto different negatives for eventual processing into three-dimensional prints. While the method of providing a viewable print from negatives so obtained is not described in this patent, only lateral camera motion is described and therefore a viewing method providing horizontal image separation is most likely. The other Wright patent (U.S. Pat. No. 4,956,705) describes the same image capture process as the '449 patent using video CCD array cameras rather than photographic cameras and further discusses capturing the images using a "frame grabber" board in a computer which freezes a frame of a still or moving object and digitizes the image for further processing, such as by software "paint" type programs.
Horizontal image separation may also be provided through raster occlusion, such as by using a Ronchi ruling on a faceplate spacially located in front of the composite print so as to prevent the images intended for viewing by the right eye to be seen by the left eye and vice versa. The technique of raster occlusion is described in textbooks such as Foundations of the Stereoscopic Cinema by Lenny Lipton (New York: VanNostrand Reinhold, 1982, pages 74, 166, 287) and Stereoscopy by N. A. Valyus (Focal Press, 1966). Compared to lenticular methods, however, raster occlusion suffers from the additional problem of reduced image brightness.
The prior methods of optically recording scenes on lenticular print material so that the angular presentations of the lenslets correspond correctly with the angular orientations of the original scene rely upon the aforementioned "indirect" printing process. In contrast, U.S. Pat. Nos. 4,552,442 and 4,674,853, naming Graham S. B. Street as inventor, teach a "direct" method of recording images with correct angular correlation. In this method, the converging bundle of optical rays from a very large aperture camera lens is directed onto a sheet of lenticular material to which photographic film has been affixed in the same manner as described in the aforementioned projection method. In optical terms, the apertures of the respective lenslets form the sub-apertures which sample the taking lens aperture. Left-right image correspondence is properly established by reflecting the converging beam from a beamsplitter onto a retroreflecting surface. In the geometric space between the object field and the taking lens, different aperture coordinates, or positions on the aperture of the taking lens, represent different perspectives of the object field. Bundles of light rays leaving the taking lens from localized sub-apertures within the taking lens aperture are relayed by different lenslets on the lenticular film array to the photographic emulsion layer for exposure. Problems include light losses from the beamsplitter and multiple optical reflections, and the need for a uniformly accurate retroreflector sheet with elements not substantially larger than the display print lenticules. Moreover, the depth of field of the camera lens severely limits the photographic space, and the camera itself is extremely large, necessitating the use of large format film sheets for each copy.
To summarize, in prior art techniques the means of increasing the number of perspectives depended on one of two basic methods:
1. The "indirect" method of photographing the subject field from different perspectives either by shifting the camera, if a single camera is being used, or by adding additional cameras at different perspectives, with a means of synchronizing the camera shutters to open at the same instant in time. When one camera is used by shifting between exposures to capture alternative perspectives, the subject field is limited to stationary objects, an unacceptable limitation to photographic freedom for the photographer. The use of multiple cameras which are synchronized for exposure solves this problem, but adds to equipment complexity and cost, particularly where a large number of views are required for autostereoscopic realism.
2. A "direct" method of photographing the subject field by employing a large aperture taking lens on the camera and sampling the light rays leaving the taking lens from different aperture coordinates by the smaller apertures represented by the retroreflector cascading onto the lenticules on the lenticular film assemblage. This system limits the space which can be recorded to the depth of field of the taking lens which in turn must be used in a wide open condition in order to "see" the object field over the entire range of aperture coordinates. This constraint also represents an unacceptable limitation on the photographer's freedom.